1. Technical Field of the Invention
The present invention relates to emission microscopy systems used in semiconductor integrated circuit (IC) failure analysis. More particularly, and not by way of any limitation, the present invention is directed to an emission microscopy system that includes a source for coherent and substantially uniform radiation or illumination.
2. Description of Related Art
Light (or photon) emission microscopy is a common failure analysis technique used for analyzing semiconductor integrated circuit (IC) devices. The considerations involved in using photon emission to successfully analyze defects and failure mechanisms in CMOS ICs are well known. IC failure analysis using an emission microscope is performed by collecting visible (390-770 nm), and sometimes near infrared (NIR) (770-1000 nm, with the typical IR band defined as 770-1500 nm), wavelength photons emitted from transistors, p/n junctions, and other photon-generating structures on or near the top (front), electrically-active, silicon surface. These photons are transmitted through the overlying, relatively transparent dielectric layers, passing between or scattered around the patterned, opaque metal interconnections. Detection of photons that emerge from around these overlying layers is referred to as frontside light emission analysis. Correspondingly, imaging light passing through the silicon substrate and emerging from the bottom (back) is referred to as backside light emission analysis.
Custom and commercial systems are routinely used for light emission analysis. The spectral characteristics for these systems are usually dependent upon the type of detector chosen. Most commercial systems use detectors based on image intensifiers or CCD arrays. Although current systems can provide detectors with extended NIR capability for backside analysis, most systems have very low response to photons with wavelengths beyond 1 .mu.m.
There is an increasing interest in backside light emission analysis. This is driven primarily by the advancement of IC fabrication technologies with additional opaque conductor layers and packaging technologies that typically obscure the active side of the die. Backside analysis takes advantage of silicon's transmission of photons with energies less than its indirect silicon bandgap energy, corresponding to wavelengths greater than around 1.107 .mu.m (for undoped silicon). It is commonly known that silicon becomes less transparent as dopants are added. Because of this phenomenon, the heavily doped substrates often used with newer technologies will attenuate NIR light emitted from the active circuits. These and other factors are stimulating research for solutions, including improved substrate thinning techniques, increased NIR imaging sensitivity, and spectral analysis.
It is well known that different types of photon emission processes can be distinguished by their spectra. Photon emission from defects or abnormal operation of silicon microelectronic devices generally falls into the following categories: forward or reverse biased p/n junctions, transistors in saturation, latchup, and gate oxide breakdown. While radiative recombination emission from silicon structures is generally centered around 1.1 .mu.m, commonly used cameras have spectral response centered in the 400-900 nm range and can thus capture only a small portion of the emitted light.
Traditional methods of NIR imaging use an optical filter in conjunction with a broad-spectrum illuminator such as a quartz halogen bulb. The desired wavelengths pass through the filter and are used in the microscope illuminating path. The desired wavelength is selected by the filter when the unwanted light frequencies are rejected. One of the problems of the current technologies is that when a more intense illumination source is used to address at least in part the issue of the poor quantum efficiency of backside emission, the optical filters get degraded or destroyed quickly due to heating. The problem is further compounded by the fact that as the filter bandwidth is narrowed, the total energy is also reduced from the source output. On the other hand, employing longer integration times, by taking the emitted light inputs over a considerable period of time, negatively impacts the through-put. Due to these constraints, it can be appreciated that the current illumination technology cannot provide intense, narrow bandwidth illumination that is highly advantageous in backside emission analysis.
Laser sources can provide very intense, substantially monochromatic illumination. When these sources are used in backside emission analysis, however, interference phenomena cause what is commonly known laser "speckle" that blur the illuminated image. The speckle is seen at least in part due to the nonuniform distribution of radiation energy, giving rise to "hot spots" and "dark areas". While techniques such as diffusing the laser light using a frosted glass, dithering (i.e., scanning the laser beam), et cetera, are sometimes used, they have not been sufficiently effective in alleviating the speckle problem in backside emission imaging. Further, it may be appreciated that the recent popularity of flipchip technologies, rapid escalation in the number of metal interconnect layers and advanced packaging techniques (for example, ball grid arrays, land grid arrays, etc.)--all of which obscure the front side view of the active area--make the need to solve the speckle problem more acute.